In the optical communications space, receivers based on coherent detection techniques have suffered disadvantages that have, to date, prevented successful deployment in “real-world” installed communications networks.
One such limitation is that both the transmitted carrier signal and the receiver's local oscillator (LO) signal are generated by respective transmitter and LO lasers, which, in the case of “real world” network systems, will be compact fiber or semi-conductor lasers which are subject to manufacturing and environmental variations. Such lasers are typically designed such that the average output frequency (over a period of 100 s of milliseconds or more) is stable at a value which is nominally fixed by the frequency setting. However, short period frequency excursions due to laser line width and phase noise are permitted. As a result, frequency variations of as much as ±400 MHz, at rates on the order of up to 50KHz are commonly encountered. The resulting frequency mismatch Δf between the LO signal and the received carrier signal appears as a phase error in recovered symbols, which can lead to erroneous data detection.
In prior art coherent receiver systems, this problem is typically addressed by implementing an optical frequency locked loop (FLL) or Phase locked loop (PLL) to actively control the receiver's LO to match the received carrier signal. FLL and PLL circuits for this purpose are described in: “High Capacity Coherent Lightwave Systems”, Linke et al, Journal of Lightwave Technology, Vol. 6, No. 11, November 1988; “Heterodyne Phase Locked Loop by Confocal Fabry-Perot Cavity Coupled AlGaAs lasers”, Shin et al, IEEE Photonics Technology Letters, Vol. 2, No. 4, April 1990; and “Carrier Synchronization for 3 and 4-bit-per-Symbol Optical Transmission”, Ip et al, Journal of Lightwave Technology, Vol. 23, No. 12, December 2005. All of these systems operate to drive the receiver's LO to precisely track excursions of the received optical carrier. A limitation of this approach is that for optical communications systems with multi-gigabit line rates, a PLL/FLL loop bandwidth on the order of hundreds of MHz is needed to effectively compensate the laser phase noise. This is difficult to achieve at acceptable cost.
An alternative approach is to use an electrical carrier recovery circuit for detecting and compensating the frequency mismatch between the LO and received carrier. A carrier recovery circuit designed for this purpose is described in “Phase Noise-Tolerant Synchronous QPSK/BPSK Baseband-Type Intradyne Receiver Concept With Feedforward Carrier Recovery”, R Noé, Journal of Lightwave Technology, Vol. 23, No. 2, February 2005. A limitation of electrical carrier compensation in this manner is that it can only feasibly compensate some aspects of moderate frequency errors. As a result, a large frequency transient can cause severe performance degradations, for example due to limited analog amplifier bandwidth, and clock recovery issues.
A further limitation of coherent detection systems is that they are highly vulnerable to optical impairments of the received carrier signal. In particular, optical signals received through conventional optical links are distorted by significant amounts of chromatic dispersion (CD) and polarization dependent impairments such as Polarization Mode Dispersion (PMD), polarization angle changes and polarization dependent loss (PDL). Chromatic dispersion (CD) on the order of 30,000 ps/nm, and polarization rotation transients at rates of 105Hz are commonly encountered.
Various methods of compensating Polarization angle are known in the art. See, for example, “Phase Noise-Tolerant Synchronous QPSK/BPSK Baseband-Type Intradyne Receiver Concept With Feedforward Carrier Recovery”, R Noé, Journal of Lightwave Technology, Vol. 23, No. 2, February 2005, and “PLL-Free Synchronous QPSK Polarization Multipex/Diversity Receiver Concept with Digital I&Q Baseband Processing”, R Noé, IEEE Photonics Technology Letters, Vol. 17, No. 4, April 2005. In this respect, it will be noted that Noé also alludes (in the introduction) to the possibility of also compensating chromatic dispersion. However, Noé does not provide any teaching as to how this would be done. The applicability of RF channel estimation techniques to the detection of polarization-division multiplexed optical signals in a quadrature coherent receiver is described by Y. Han et al. in “Coherent optical Communication Using Polarization Multiple-Input-Multiple-Output”, OPTICS EXPRESS Vol. 13, No. 19, pp 7527-7534, 19 Sep. 2005.
A limitation that is common throughout the prior art is a lack of satisfactory bandwidth of the various compensation functions. For example, the FLL/PLL and carrier recovery techniques described above are intended to track (and thus compensate) laser phase noise. However, in order to provide sufficient accuracy of compensation, they lack sufficient bandwidth to acquire a signal across the entire possible range of impairment magnitude, such as a frequency error of several GigaHertz. As a result, these systems cannot reliably acquire a signal and stabilize to steady-state operation, even if they could track laser phase transients after a steady state had been achieved. Similarly, the system of Noé [supra] is designed to compensate polarization rotations, but it cannot track high speed transients of the type encountered in real-world communications networks. For example, Noé, claims that with a 10 GBaud signal, the inverse Jones matrix coefficients can be updated with a period of 16 μs. This is far too slow to successfully compensate 20 kHz polarization rotations, which have a period of 50 μs. In addition, the system of Noé tends to fail in the presence of severe Chromatic Dispersion (CD), at least in part due to failure of the clock recovery circuit as inter-symbol interference (ISI) increases, and consequent uncertainty of the sample timing of the A/D converters. While it is mathematically possible to design a filter function that compensates both polarization and chromatic dispersion (as alluded to by Noé), the prior art does not offer any methods by which satisfactory compensation accuracy can be obtained with an adaptation speed high enough to track real-world polarization transients. It follows that the system of Noé will not be able to reliably capture the instantaneous polarization state of the received signal during start-up, especially in the presence of high speed transients, and thus cannot guarantee that it will achieve a stable steady-state operation.
Prior art clock recovery systems suffer the same limitation, in that the PLL bandwidth required to obtain a satisfactory sample phase accuracy is significantly less than the possible range of clock and channel errors. As a result, conventional clock recovery circuits cannot reliably acquire a lock condition, even if they are able to maintain lock once it has been achieved. A further limitation of clock recovery circuits is that they are very vulnerable to distortions in the received optical signal. While this can be overcome by compensating at least some of the distortions prior to the clock recovery circuit, such compensation normally requires the recovered clock signal in order to operate. As a result, the receiver cannot reliably acquire signal and achieve a steady state operation, even if such a state can be maintained once it has been achieved.
Accordingly, methods and techniques that enable a coherent optical receiver to reliably acquire signal and achieve steady-state operation remain highly desirable.